Marijuana is the most commonly abused illicit substance in the United States and therefore its health consequences are of considerable significance. Postural dizziness and syncope have been reported after marijuana smoking. In a pilot study we conducted, 6 out of 10 subjects reported moderate to severe dizziness after smoking one marijuana cigarette (THC content 3.55%). This project studies the frequency, severity and duration of postural syncope after smoking marijuana cigarettes of 2 strengths. It also examines the physiological basis for the post-marijuana postural syncope by studying changes in middle cerebral artery velocity, blood pressure, forehead skin perspiration and plasma levels of catecholamines. 70 physically and mentally healthy adults of both sexes with a history of previous exposure to marijuana will be recruited. The cerebral blood velocity and the indices mentioned above will be measured in the supine position (lO minutes), while standing up (lO minutes) and lying down again (lO minutes) before and four times after: immediately after, 30 minutes, 6O minutes, 90 minutes and 120 minutes after) smoking marijuana cigarettes of 2 strengths (THC 1.75% and 3.55%) and a marijuana placebo cigarette (marijuana with THC extracted) over a period of 2 hours. The 2 marijuana cigarettes and the placebo cigarette will be given during 3 separate visits to the laboratory separated by a minimum of one week, under double-blind conditions. Plasma level of THC, pulse rate, end tidal carbon dioxide, and end tidal carbon monoxide will be continually monitored during this period. Severity of dizziness when the subject stands up will also be quantified with a writing instrument.